Why Amazon.com is a credible threat to Google

Just when you think you're safe, that's when the competition swoops in and proves otherwise. This could be the fate awaiting Google (NAS: GOOG). An alarming 97% of the search giant's $37.9 billion in revenue last year came from advertising. Google relies on healthy ad sales to stay in business -- a livelihood that Amazon.com (NAS:AMZN) could threaten.